1. Field of the Invention
The present invention relates generally to methods for enabling relatively rapid image acquisition through the collection of magnetic resonance signals using a scanning sequence having cycles or sub-cycles with a duration typically of the same order of magnitude as the longitudinal and/or transverse relaxation time constants of a material of interest. The present invention also relates to scanning sequences composed of two or more sub cycles or modes.
2. Description of the Prior Art
The desire to increase productivity of medical diagnostic magnetic resonance imaging (MRI) equipment has led to the development of numerous rapid scanning techniques for reducing the duration of MRI examinations. Typically, each cycle of a scanning sequence has a duration T.sub.R on the order of the longitudinal relaxation time constant T.sub.1 of the material of interest, each cycle having an RF excitation pulse characterized by an angle .alpha. (the magnetic flip angle caused by the pulse) which is substantially optimized for signal to noise ratio as a known function of T.sub.R and T.sub.1. Because the precessing nuclear resonance spins excited by each RF excitation pulse do not relax to insignificance before the next RF excitation pulse, there arises an interaction between successive cycles such that a steady state free precession spin ensemble is formed from which gradient echoes are excited during the sequences. Typical fast imaging methods are discussed in P. van der Meulen, et al, "FAST FIELD IMAGING: An Overview and Contrast Calculations", Magnetic Resonance Imaging, Vol. 6, pp. 335-368, 1988. While typical fast imaging techniques have utilized a constant period T.sub.R between excitation pulses, a different class of sequences, with no single interpulse period, designated SYNCOPATED PERIODIC EXCITATION (SPEX) has been considered to form a steady state spin ensemble. W. Sattin, "SYNCOPATED PERIODIC EXCITATION (Spex): Fast Imaging With Reduced susceptibility Artifacts" SMRI 1988: SIXTH ANNUAL MEETING PROGRAM AND ABSTRACTS No. 203 and S. Patz et al, "MISSING PULSE STEADY STATE FREE PRECESSION", Magnetic Resonance in Medicine 10, pp. 194-209 (1989) have proposed sequences having two different interpulse periods of durations T and 2T.
It is known in conventional magnetic resonance imaging that the image formed from collected magnetic resonance signals represents a spatial distribution of nuclear magnetization spin densities. The spin density at each point in space may be a combination of contributions from physically different spin populations having different characteristics such as transverse and/or longitudinal relaxation. The areas of contrast in an image may therefore be varied by emphasizing or deemphasizing the contributions from the physically different populations. This emphasis or deemphasis is a weighting due to the various parameters of a sequence such as the time T.sub.E after an excitation pulse when an echo is excited. Other weightings are possible to attempt to emphasize such physical characteristics as the relative Larmor frequency of the population, the flow or diffusion of the population or the degree of coupling of the population to other excited spin systems. Furthermore, comparisons between differently weighted images provide the potential for further types of contrast emphasis or deemphasis.
The essentially identical techniques proposed by Lee and Cho, Magnetic Resonance in Medicine 8, 142-150 (1988) and by Redpath and Jones, Magnetic Resonance in Medicine 6, 224 (1988) contemporaneously generate two separate images from the free induction decay (FID) signal and a "contrast enhanced" echo signal in a conventional "single-mode" steady state free precession sequence. However, the range of Weights for this pair of images is relatively limited, generally not allowing sufficient flexibility to substantially isolate dependence on any particular distinct parameter.